The present invention relates to wireless digital communications, and more particularly to space diversity transmission systems and methods.
Wireless communication systems include a large variety of approaches, such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and combinations. FDMA uses separate frequency bands for duplex communication; whereas, TDMA partitions a single frequency band into time slots which as allocated to one or the other end of a communication link. CDMA uses a spread spectrum approach.
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for “spreading” information to encode it and for “despreading” (by correlation) received spread spectrum signals and recovery of information. Such multiple access typically appears under the name of code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while other users' interfering signals appear noise-like. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a pseudo-random code pulse (chip) interval TC of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The recent 3GPP wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 256 chips. Indeed, UMTS (universal mobile telecommunications system) approach UTRA (UMTS terrestrial radio access) provides a spread spectrum cellular air interface with both FDD (frequency division duplex) and TDD (time division duplex) modes of operation. UTRA currently employs 10 ms duration frames partitioned into 15 time slots with each time slot consisting of 2560 chips (TC=0.26 microsecond).
The air interface leads to multipath reception, so a RAKE receiver has individual demodulators (fingers) tracking separate paths and combines the finger results to improve signal-to-noise ratio (SNR). The combining may use a method such as the maximal ratio combining (MRC) in which the individual detected signals in the fingers are synchronized and weighted according to their signal strengths or SNRs and summed to provide the decoding. That is, a RAKE receiver typically has a number of DLL or TDL code tracking loops together with control circuitry for assigning tracking units to the strongest received paths. Also, an antenna array could be used for directionality by phasing the combined signals from the antennas.
Further, UTRA allows for transmit diversity, both open-loop and closed-loop (receiver feedback). The open-loop transmit diversity includes both time-switched transmit diversity (TSTD) and space-time block-coding-based transmit diversity (STTD). Closed loop techniques provide some significant gain over open-loop transmit diversity techniques by using channel state information (CSI) at the transmitter. For FDD the CSI can be made available at the transmitter via a feedback channel; whereas, for TDD the channel can be directly measured at the transmitter by exploiting the reciprocity (uplink and downlink using the same channel).
FIG. 2d illustrates a generic closed-loop diversity transmitter with P antennas; the feedback channel information determines the weightings among the transmit antennas based on SNRs. With FDD the receiver determines the weightings and signals the transmitter. The spreading blocks of FIG. 2d apply for CDMA systems.
For CDMA with large spreading factors (e.g., 256), the channel delay profile is very small compared to one symbol period. This allows a system to exploit multipath diversity without having to suffer from multipath interference (ISI and MUI). FIG. 3c illustrates a receiver for this situation in which the weightings derive from maximizing the received SNR based on the maximal ratio combining (MRC) or Rake receiver. Note that this received SNR metric neglects the effect of multipath interference, i.e., a Rake receiver successfully separates multiple copies of the signal that arrive at different times.
However, high data rate CDMA schemes, such as HSDPA and 1xEV-DV, use a small spreading factor (e.g., 16) and consequent small symbol period. Hence, multipath interference is not negligible and such high data rate schemes demand equalization or interference cancellation which disrupts multi-antenna weighting determinations. As a result, the weight vector selection based on MRC or Rake receiver is sub-optimal since such criterion assumes perfect multipath interference suppression.